The present invention pertains to apparatus for sealing leaks between a casing and a casing head and more particularly, to a seal which intensifies the pressure inside the casing seal to create improved sealing action.
Crude oil and natural gas wells drilled offshore often have their wellheads positioned on the ocean floor. The wellhead is normally installed by mounting it on the upper end of a string of casing which is lowered into the bore hole and cemented in place. The wellhead is generally in the shape of a cylinder having a bore which is adapted to sealably support an inner concentric string of casings and casing hangers that are affixed to the casing string prior to lowering the string and seating the hanger within the wellhead. The casing hangers are adapted to be received and supported within the wellhead. Each casing hanger is provided with a packoff element around its outer periphery to provide a pressure-tight seal between the outside of the casing and the inside of the wellhead.
In many present day well completion systems annular packoff seal elements are positioned between the wellhead and each casing to provide a pressure-tight seal therebetween. It is important that gas, water and other fluids which may be forced into some of the outer casings be prevented from moving into the inner casing and that fluids from the inner casing be prevented from moving into the outer casings. Each of the packoff seals must be designed to withstand the maximum oil pressure since pressure abnormalities are controlled by closing a blowout preventer situated above the wellhead, and this will cause high pressure to be exerted across the various seals.
Some of the prior art seals include an elastomeric material which is compressed by a high pressure exerted against the elastomeric material. This high pressure forces the elastomeric material against the casing and against the wellhead to provide a seal between the casing and the wellhead. This pressure can cause gases to be forced into the elastomeric material. Any subsequent release of the pressure allows the gases to rapidly move out of the material which causes shredding and destruction of the material and causes the seal to fail.